Zostavax and Medicare and GA or GX modifier

I am confused about this issue. I know that Medicare Part B doesn't cover the Zostavax vaccine. But I think Medicare Part D does. My doctor got an ABN signed and I want to bill for the vaccine and the administration of the vaccine. Do I use a Ga (Waiver of Liability Statement issued as required by payer policy) or do I use the new modifier GX (notice of liability issued, valuntary uner payer policy) to be used to report when a voluntary ABN was issued for a service? I am leaning towards the new GX modifier, but I would really appreciate any advice on this. I have the MLN Matters from CMS for 2/19/10 effective date 4/1/10 number MM6563 revised, but I am uncertain as to which modifier applies. Please help.

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